CLINICAL CORE: The Clinical Core will recruit and follow all the subjects for the four projects that compose this P01 grant. The Clinical Core will build on a very successful research program on early HIV infection, the UCSF Options Project, which has been led by Dr. Hecht since its inception in 1996 and constitutes an exceptional resource for the proposed studies. The Options Project has also established successful methods for recruiting potential source partners of newly infected persons and has identified over 40 partner pairs with confirmed phylogenetic evidence of linkage. The Clinical core will also incorporate a primary infection cohort in Caxias do Sol, located in southern Brazil, which will provide focused recruitment of recently HIV infected persons who can identify potential source partners. This region of Brazil has an HIV infection in which there is a mixture of similar frequencies of subtype B and C virus, offering a resource for comparing these important HIV subtypes in the research projects. The Clinical Core will be responsible for recruiting and following persons with HIV infection of less than 6 months' duration for this grant. In San Francisco, we aim to enroll 50 persons per year, and 30 potential source partners for initial transmission of HIV. In Caxias do Sol, we aim to enroll 25 recently infected persons per year, and 25 potential source partners. Potential source partners will be evaluated using phylogenetic sequencing to identify likely source partners. Follow-up will be performed on subjects who meet key criteria for the projects in this proposal. The core will obtain biological specimens for the research projects and process and store them as needed. It will also gather clinical data that will be used by each of the projects, including clinical status, and core laboratory data such as viral load and CD4 T cell counts. The Clinical Core will leverage UCSF-GIVI Center for AIDS Research (CFAR) resources to provide more specialized laboratory data on participants that will be used across projects, including resistance genotyping (CFAR Core Virology Laboratory) and immunophenotyping for activation markers (CFAR Core Immunology Laboratory). Specimen processing and cryopreservation at UCSF will be performed by the UCSF-CFAR AIDS Specimen Bank, which has extensive experience in processing and storing biological specimens for HIV studies. The Brazil component will build on an established infrastructure for HIV related cohort studies, and incorporate expert virology and immunology labs at the Federal University Medical School in Sao Paulo. The clinical data and biological specimens generated by the Clinical Core will also form an important research resource for investigators within and outside of UCSF.